Since 1993, environmental legislation in the U.S. has required that the sulfur content of diesel fuel be less than 0.05%. The reduction in the sulfur content of diesel fuel has resulted in lubricity problems. It has become generally accepted that the reduction in sulfur is also accompanied by a reduction in polar oxygenated compounds and polycyclic aromatics including nitrogen-containing compounds responsible for the reduced boundary lubricating ability of severely refined (low sulfur) fuels. While low sulfur content is not in itself a lubricity problem, it has become the measure of the degree of refinement of the fuel and thus reflects the level of the removal of polar oxygenated compounds and polycyclic aromatics including nitrogen-containing compounds.
Low sulfur diesel fuels have been found to increase the sliding adhesive wear and fretting wear of pump components such as rollers, cam plate, coupling, lever joints and shaft drive journal bearings.
Concern for the environment has resulted in moves to significantly reduce the noxious components in emissions when fuel oils are burnt, particularly in engines such as diesel engines. Attempts are being made, for example, to minimize sulfur dioxide emissions by minimizing the sulfur content of fuel oils. Although typical diesel fuel oils have in the past contained 1% by weight or more of sulfur (expressed as elemental sulfur) it is now considered desirable to reduce the level, preferably to 0.05% by weight and, advantageously, to less than 0.01% by weight, particularly less than 0.001% by weight.
Additional refining of fuel oils, necessary to achieve these low sulfur levels, often results in a reduction in the levels of polar components. In addition, refinery processes can reduce the level of polynuclear aromatic compounds present in such fuel oils.
Reducing the level of one or more of the sulfur, polynuclear aromatic or polar components of diesel fuel oil can reduce the ability of the oil to lubricate the injection system of the engine. As a result of poor fuel lubrication properties the fuel injection pump of the engine may fail relatively early in the life of an engine. Failure may occur in fuel injection systems such as high-pressure rotary distributors, in-line pumps and injectors. The problem of poor lubricity in diesel fuel oils is likely to be exacerbated by future engine developments, aimed at further reducing emissions, which will result in engines having more exacting lubricity requirements than present engines. For example, the advent of high-pressure unit injectors increases the fuel oil lubricity requirement.
Similarly, poor lubricity can lead to wear problems in other mechanical devices dependent for lubrication on the natural lubricity of fuel oil.
Lubricity additives for fuel oils have been described in the art. WO 94/17160 describes an additive, which comprises an ester of a carboxylic acid and an alcohol, wherein the acid has from 2 to 50 carbon atoms and the alcohol has one or more carbon atoms. Glycerol monooleate is an example. Although general mixtures were contemplated, no specific mixtures of esters were disclosed.
U.S. Pat. No. 3,273,981 discloses a lubricity additive being a mixture of A+B wherein A is a polybasic acid, or a polybasic acid ester made by reacting the acid with C1-C5 monohydric alcohols; while B is a partial ester of a polyhydric alcohol and a fatty acid, for example glyceryl monooleate, sorbitan monooleate or pentaerythitol monooleate. The mixture finds application in jet fuels.
U.S. Pat. No. 6,080,212 teaches of the use of two esters with different viscosity in diesel fuel to reduce smoke emissions and increase fuel lubricity. In one preferred embodiment of that invention methyl octadecenoate, a major component of biodiesel, was included in the formula. Similarly, U.S. Pat. No. 5,882,364 also describes a fuel composition comprising middle distillate fuel oil and two additional lubricating components. Those components being (a) an ester of an unsaturated monocarboxylic acid and a polyhydric alcohol and (b) an ester of a polyunsaturated monocarboxylic acid and a polyhydric alcohol having at least three hydroxy groups.
The approach of using a two component lubricity additive was pioneered in U.S. Pat. No. 4,920,691. The inventors describe an additive and a liquid hydrocarbon fuel composition consisting essentially of a fuel and a mixture of two straight chain carboxylic acid esters, one having a low molecular weight and the other having a higher molecular weight.
In U.S. Pat. No. 5,713,965 the synthesis of alkyl esters from animal fats, vegetable oils, rendered fats and restaurant grease is described. The resultant alkyl esters are reported to be useful as additives to automotive fuels and lubricants.
Alkyl esters of fatty acids derived from vegetable oleaginous seeds were recommended at rates between 100 to 10,000 ppm to enhance the lubricity of motor fuels in U.S. Pat. No. 5,599,358. Similarly a fuel composition was disclosed in U.S. Pat. No. 5,730,029 comprising low sulfur diesel fuel and esters from the transesterification of at least one animal fat or vegetable oil triglyceride.